Generally, the increasing use wireless communication systems has led to more dense utilization of the wireless spectrum, bringing adjacently-used frequency bands closer together. The use of pass-band filters is not ideal, as such filters may not be capable of filtering-out interference in the adjacent frequency bands. Moreover, receivers can suffer various interferences from internal and external transmit signal leakage, and these interferences can adversely affect duplexers or receiving filters.
Acoustic wave filters, such as surface acoustic wave (SAW) filters or bulk acoustic wave (BAW) filters, can be used to filter out interference, such as between frequency bands. An acoustic wave filter circuit generally includes multiple acoustic wave resonators, which convert electrical signals into acoustic waves, and vice versa. A SAW resonator typically includes a piezoelectric crystal or ceramic material with input and output interdigital transducers (IDTs) formed in a thin film metal on the piezoelectric material, where the acoustic wave propagates on the surface of the piezoelectric material. A BAW resonator typically includes a piezoelectric material with two electrodes formed on either side of the substrate, where the acoustic wave propagates through the bulk of the piezoelectric material. Piezoelectric materials include, for example, aluminum nitride, zinc oxide, quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, lead zirconate titanate. Various piezoelectric materials, such as quartz, may be manufactured with a selected cut angle, such as AT-cut, ST-cut, Y-cut, and rotated Y-cut, each of which provides different operating characteristics for the resonator.
Acoustic wave devices may be fabricated, for example, with standard integrated circuit technology. A bulk piezoelectric material may be used as the substrate. Alternatively, insulating silicon, or a silicon dioxide layer over silicon semiconductor, may be used as the substrate. If silicon is used, a piezoelectric material layer, such as zinc oxide, is formed on the substrate. The IDTs may be formed on or under the piezoelectric layer using, for example, an integrated circuit etching or lift-off process. In the etching process, a metal layer is formed on the device, and photolithography is used to positively pattern the IDTs and remove the unwanted metal from the device. In the lift-off process, photolithography is used to negatively pattern the IDTs, a metal layer is deposited, and the unwanted metal is lifted-off of the device.
The foregoing structures and fabrication processes are disadvantageous, as they fail to attain the requisite filtering performance called for in contemporary wireless networks, particularly given the increasingly dense use of adjacent frequency bands. Moreover, the known filter structures are not scaled desirably for contemporary networks.